The present invention relates to fuel cells which are useful for cogeneration systems and mobile apparatuses. More specifically, it relates to polymer electrolyte fuel cells.
Fuel cells generate electric power and heat simultaneously by electrochemically reacting a fuel such as hydrogen and an oxidant gas such as air at a gas diffusion electrode, and there are several types different with respect to the type of electrolyte used and the operating temperature. In polymer electrolyte fuel cells, it is mainstream to use as a polymer electrolyte, fluorocarbon polymer with a sulfonic acid group introduced as a side chain terminal group. An electrode reaction layer which is mainly composed of a carbon powder with a platinum group metal catalyst carried thereon is formed on each side of the electrolyte layer. A pair of electrically conductive porous sheet material which has both gas permeability and electric conductivity is formed on the outer surfaces of the electrode reaction layers. This combination of the electrically conductive porous sheet material and the electrode reaction later is referred to as a gas diffusion electrode.
Provided on the outer surface of the gas diffusion electrodes are electrically conductive separator plates for mechanically fixing these electrolyte-electrode assemblies and for electrically connecting in series the adjacent assemblies. On the surface of the separator plates contacting the electrodes, gas flow paths are formed for supplying reaction gases to the electrode surfaces and for transporting a generated gas and an excess gas away. In the periphery of the separator plates and the electrodes, sealing members such as gaskets and sealants are provided for preventing two reaction gases from being directly mixed or leaking out.
To one of the pair of gas diffusion electrodes, a gas containing hydrogen as a reaction gas is generally supplied. However, organic liquids such as alcohol and ether are used in some cases. To the other gas diffusion electrode, an oxidant gas such as air is supplied.
In the system where a fuel gas is used as a reaction gas, it is necessary to provide a reformer for transforming organic fuel substances such as methane gas, propane gas and alcohol into gases containing hydrogen richly by reforming reaction such as partial combustion and hydrogenation.
A power generator is usually constructed as a stacked cell in which a plurality of unit cells comprising the electrolyte layer, electrode reaction layers, separator plates and the like are stacked, and in which a fuel gas such as hydrogen and air are supplied to the gas flow path of each unit cell through manifolds. Electric current generated in the electrode reaction layers is collected in the electrically conductive porous sheet material and is taken outside through the separator plates. For the separator plates, a carbon material which is electrically conductive and has both gas tightness and corrosion resistance is often used. However, metallic separator plates such as stainless steel are also used in view of its good processability and inexpensiveness, and also from the viewpoint that thinner separator plates can be obtained
Since heat is generated during power generation utilizing the electrochemical reaction, cooling water or antifreezing fluid is allowed to flow inside the cell to control the cell temperature. In general, heated cooling water is cooled by a heat exchanger disposed outside the cell and flows again to the inside of the cell.
In the case where a fluorocarbon polymer with a sulfonic acid group introduced therein is used as the electrolyte, if organic liquids such as alcohol is used as the reaction gas, these organic liquids permeate through the fuel electrode and the electrolyte layer to reach the air electrode in the opposite side. As a result, in the air electrode, organic fuel substances cause direct catalytic combustion with oxygen contained in the air to impair the cell performance. Also, in the fuel electrode, a decrease in the cell performance occurs presumably due to CO poisoning as a result of the electrode reaction.
On the other hand, in the case where a reformed gas formed in a reformer is used as the reaction gas, the electrode is poisoned by CO2 and CO contained in a small amount during a long duration of operation, and thereby the cell performance is impaired. Further, from the viewpoint of the fuel cell system, the fact that a reformer must be provided separately results in complication of the system and an increase in the cost.
The principal object of the present invention is to provide a direct methanol type fuel cell having an improved cell performance and durability.
Another object of the present invention is to provide a fuel cell using as the fuel a reformed gas containing hydrogen and having an improved resistance to CO poisoning.
The present invention provides a fuel cell system comprising:
a first electrolyte-electrode assembly comprising a hydrogen ion-conductive electrolyte layer, and a fuel electrode and a hydrogen-generating electrode sandwiching the electrolyte layer;
a second electrolyte-electrode assembly comprising a hydrogen ion-conductive electrolyte layer, and a fuel electrode and an oxidant electrode sandwiching the electrolyte layer;
a fuel supplying means for supplying a liquid or gas fuel to the first electrolyte-electrode assembly;
a means for applying to the first electrolyte electrode assembly a potential which is positive to the hydrogen-generating electrode; and
a means for supplying to the fuel electrode of the second electrolyte-electrode assembly hydrogen generated in the hydrogen-generating electrode.
Herein, it is preferable that the hydrogen-generating electrode of the first electrolyte-electrode assembly and the fuel electrode of the second electrolyte-electrode assembly are electrically connected. In this case, it is more preferable that the hydrogen-generating electrode of the first electrolyte-electrode assembly and the fuel electrode of the second electrolyte-electrode assembly are unified. In another preferred mode, the hydrogen-generating electrode of the first electrolyte-electrode assembly and the fuel electrode of the second electrolyte-electrode assembly are combined with a water repellent, electrically conductive layer interposed therebetween.
As a means for applying to the first electrolyte-electrode assembly a potential which is positive to the hydrogen-generating electrode, the second electrolyte-electrode assembly is preferably used.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.